This subject matter disclosed in this application relates generally to components for gas turbine engines, and more particularly, to an apparatus and method for suppressing dynamic pressure instability in a bleed duct of a gas turbine engine.
Bleed ducts on gas turbine engines extract a portion of the air from the flow path of the fan bypass duct and direct the air through a bleed duct to a heat exchanger to cool the engine. The flow of air through the bleed duct is typically controlled by a flow control valve, for example a butterfly valve. The flow control valve is closed during certain modes of engine operation, thereby preventing the flow of air through the bleed duct. When the valve is closed (or the amount of air leaking through the valve is sufficiently small) and air is flowing at a high rate through the fan bypass duct past the inlet of the bleed duct, the bleed duct may become dynamically unstable and undergo pressure oscillations of high magnitude. Dynamic pressure levels as high as 1.125 kg/cm (16 psi) peak-to-peak at a characteristic (i.e. natural) frequency have been observed, which can cause significant structural damage to the surfaces on which the dynamic pressure loading occurs. As a result, high dynamic pressure levels in the bleed ducts of gas turbine engines must be avoided.
The noise generation mechanism of a Hartmann Generator is governed by the same physical phenomena exhibited by a dynamically unstable bleed duct in a gas turbine engine. The Hartmann Generator consists of a circular nozzle for issuing an air jet that is operatively coupled with a relatively short, circular tube having an open end adjacent the nozzle and a closed end opposite the open end and the nozzle. The open end of the tube has a diameter approximately equal to the diameter of the air jet and is positioned only a few nozzle diameters downstream from the nozzle exit. The nozzle is oriented such that the air jet flows directly into the open end of the tube. As a result, high energy acoustic waves of a characteristic frequency are generated in the region between the nozzle exit and the open end of the tube.
The inlet of the bleed duct is positioned in a curved wall section of the fan bypass duct that allows a portion of the air flow through the bypass duct to enter the bleed duct essentially parallel to the longitudinal axis of the bleed duct. During certain modes of engine operation, a portion of the air flow through the fan bypass duct is routed directly into the inlet of the bleed duct such that high magnitude pressure oscillations result when the flow control valve is closed, or is leaking an insufficient amount of air. The usual method of avoiding dynamic pressure instability in gas turbine engines is to reduce the magnitude of the pressure oscillations by permitting the flow control valve to leak a sufficient amount of air. Recent engine operational requirements, however, do not permit valve leakage, and therefore, can cause unacceptably high dynamic pressure oscillations. In turn, the high dynamic pressure oscillations result in sonic fatigue damage and the failure of structural parts subjected to increased dynamic pressure loading, such as the surface of the bleed duct.
Like the Hartmann Generator, the physical mechanism behind the dynamic pressure instability is the periodic injection, pressurization and ejection of air from the bleed duct when the flow control valve is fully closed. The flow of air directly into the inlet of the bleed duct increases the air pressure and condenses the air in the bleed duct. The air is compressed similar to a spring until a resisting back pressure exceeds the air pressure and expels the air back out of the inlet. The expelled air diverts the air attempting to enter the inlet around the bleed duct. When the back pressure inside the bleed duct is sufficiently reduced, injection and pressurization resume until the ejection cycle is repeated. The bleed duct has a characteristic one-quarter wavelength organ pipe acoustic resonance, and the oscillating pressure fluctuations tend to couple in a non-linear, unstable fashion with the acoustic resonance, such that the frequency of pressure oscillations is related to, but not exactly equal to, the predictable acoustic resonance frequency of the bleed duct.
A second dynamic pressure instability mechanism that also results in acoustic resonances occurs when the air flow through the fan bypass duct grazes tangentially over the inlet of the bleed duct as opposed to entering the inlet directly. This second mechanism operates on bleed ducts having an inlet that defines an opening tangent to the flow of air through the fan bypass duct, or in other words, flush with the fan duct surface. The dynamic pressure instability resulting from this tangential flow mechanism is generally smaller in amplitude than the corresponding instability resulting from the Hartmann Generator mechanism.
Accordingly, there is a specific need for an apparatus for suppressing dynamic pressure instability in a bleed duct of a gas turbine engine when a flow control valve is fully closed.
There is also a specific need for a method of suppressing dynamic pressure instability in a bleed duct of a gas turbine engine when a flow control valve is fully closed.
There is a further specific need for an apparatus and method for reducing the magnitude of dynamic pressure oscillations within a bleed duct of a gas turbine engine, so as to avoid sonic fatigue damage and failures of structural parts subjected to dynamic pressure loading, without permitting a flow control valve to leak.